Local Hole

The Local Hole is a large underdense region in the nearby universe, inferred from the unexpectedly low number of galaxies observed around the Local Group.

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Galaxy luminosity function: A statistical description of how many galaxies occur at each luminosity. It helps assess whether the Local Hole contains fewer galaxies than expected across luminosities.
Local Group: The galaxy group containing the Milky Way, Andromeda, and dozens of smaller galaxies. The reported deficit is measured in the surrounding region relative to this nearby group.
Local Void: A nearby underdense region adjoining the Local Group and the Local Sheet. It is a distinct nearby void, not simply another name for the Local Hole.
Hubble tension: The discrepancy between local measurements of the Hubble constant and values inferred from the early universe. A substantial nearby underdensity has been proposed as one influence on local expansion measurements.
Galaxy redshift survey: An astronomical survey that measures galaxy positions and redshifts to map their distribution in space. Redshift surveys provide the three-dimensional galaxy counts used to identify nearby underdensities.
Virgo Supercluster: The large-scale concentration of galaxies that includes the Local Group and the Virgo Cluster. Its position provides a local reference for mapping surrounding galaxy density.
KBC void: A proposed large underdensity centered near the KBC supercluster, inferred from observations of local matter and galaxies. It is a related but separately defined claim about a much larger local underdensity.
Hubble constant: The present-day rate of cosmic expansion, usually denoted H₀. Local density can affect measurements based on nearby galaxies and their recession speeds.
Peculiar velocity: An object's motion relative to the uniform expansion of the universe. Local motions complicate distance estimates used to place galaxies around the proposed void.
Hubble's law: The relation between a galaxy's cosmological redshift and its distance in the expanding universe. It underlies many distance estimates for galaxies used in local density studies.